AU: interpreting kAudioUnitProperty_SupportedNumChannels & _BusCount in modular hosts?
AU: interpreting kAudioUnitProperty_SupportedNumChannels & _BusCount in modular hosts?
- Subject: AU: interpreting kAudioUnitProperty_SupportedNumChannels & _BusCount in modular hosts?
- From: "Ross Bencina" <email@hidden>
- Date: Tue, 18 May 2010 19:41:16 +1000
Hi
I'm trying to work out what channel combinations to offer channel-polyvalent
AUs in for a modular/patchable host. In this context the host doesn't have a
fixed bus structure so there's no context which imposes constraints on AU
channel usage. I assume that most channel-polyvalent AUs (assuming any
exist?) are designed to fit into specific usage patterns -- so I want to
provide them more-or-less as they expect to be used.
I could (in theory) offer all combinations from 1-256 channels in each
direction, but I'm not sure if (1) AU's are going to like that and (2)
whether it's worth it.
I've read the section on kAudioUnitProperty_SupportedNumChannels here:
http://developer.apple.com/mac/library/documentation/MusicAudio/Conceptual/AudioUnitProgrammingGuide/TheAudioUnit/TheAudioUnit.html
I understand what the wildcard values mean in AUChannelInfo. But it's all a
bit semantics neutral. I don't really understand when they might be used in
practice (especially the -1,-2 case). Can anyone give some examples please?
Most of the AU's I've tried either return -1,-1 or a fixed number of
channels.
At the moment I'm doing the following. Does that seem about right?
-1,-1 => offer Stereo and Mono versions
positive x and y => offer a single version with x input and y outputs
positive x, -1 => offer stereo and mono versions with x inputs (or outputs).
-n, -m => offer a single abs(n) abs(m) version
Should I even care about this? do most AU's return specific channel counts
or -1,-1?
On a related note, is kAudioUnitProperty_BusCount > 1 commonly used? I
wonder how this is intended to work when multiple buses are polyvalent?
I suspect I'm missing something about the underlying semantic model, and am
thus confused. Any thoughts?
Thank you
Ross.
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